It is known to provide low-voltage circuit breakers with overcurrent releases, which can also include protection against ground faults in addition to protection against overloads and short circuits. The determination of the ground-fault current can take place via the formation of the phasor sum of the phase currents or else the ground-fault current flowing back towards the feed is measured by means of a transducer. In this case, however, only the ground-fault current flowing through the circuit breaker (unrestricted earth fault) can be determined.
In installation applications, however, it may be necessary to also determine a ground fault between the feed (medium-voltage transformer) and the circuit breaker (restricted earth fault) in addition to the ground-fault current flowing through the low-voltage circuit breaker.